In usual, separation membranes used for nano-filtration separation or reverse osmosis separation are prepared by applying film-forming solutions onto porous supports.
Such separation membranes are categorized into two types: one including a porous support and a functional film placed only on a face of the porous support and the other including a porous support and functional films placed on both faces of the porous support. Both types of separation membranes have advantages and disadvantages. In general, the former has a disadvantage that the functional film is apt to be peeled off because the adhesion between the functional film and the porous support is weak, and the latter has a disadvantage that separation/removal performance is low when used in the form of an element or a module, because the thickness is large. In recent years, there has been a need for an increase in the membrane area of modules including such separation membranes. Since the membrane area per volume is increased in inverse proportion to the thickness of the separation membranes, the former type of separation membrane that can be reduced in thickness has become dominant.
In the separation membrane including the functional film placed only on a face of the porous support, the adhesion between the functional film and the porous support can be enhanced by allowing a film-forming solution to permeate the porous support sufficiently. However, when an excessive amount of the film-forming solution is allowed to permeate the porous support, the film-forming solution reaches a rear face of the porous support. The film-forming solution present on the rear face soils a film-forming apparatus and therefore causes the following problems: defects are formed in a subsequent separation membrane and/or a surface of an adjacent separation membrane is deteriorated when it is coiled.
Japanese Examined Patent Application Publication No. 4-21526 discloses a technique for solving the above problems, that is, a technique in which a film-forming solution is applied onto a porous support having a bilayer structure consisting of a rough front layer and a dense rear layer. In this technique, the adhesion between the functional film and the porous support is enhanced by allowing the solution to permeate the front layer sufficiently and the solution is prevented from reaching a rear surface of the porous support by the dense rear layer.
However, in the technique, the porous support has a large thickness; hence, a separation membrane having a small thickness.cannot be prepared, although such a separation membrane has been recently demanded.